The invention relates to the general field of permanent magnet synchronous machines, and more particularly to controlling permanent magnet synchronous machines for variable-speed fans.
Variable-speed fans generally include an inverter and a motor that is a permanent magnet synchronous machine. Permanent magnet synchronous machines are generally powered by means of a direct current (DC) power supply via the inverter arranged between the DC power supply and the permanent magnet synchronous machine.
To control a permanent magnet synchronous machine, i.e. to enable the motor to be controlled automatically, it is necessary to have information about the position of its rotor.
Various methods of controlling the inverter of the motor already exist, with the most common being as follows:                sinewave type control of the inverter of the motor, which requires continuous information at high resolution about the position of the rotor; and        square wave type control of the inverter of the motor or indeed 120° or “trapezoidal” type control, for which information about the position of the rotor need only be discrete and at low resolution.        
Trapezoidal control, also known as “120°” control, uses three Hall effect sensors to detect the angular position of the rotor in six positions. The three sensors serve to subdivide one electrical period of the rotor having a duration of 360° electrical, discretely into six electrical sectors each of 60° electrical. Trapezoidal control presents six operating points, each corresponding to one angular sector of the rotor. Knowing the angular sector in which the permanent magnet(s) of the rotor is/are located, the inverter powers the two appropriate phases of the stator of the synchronous machine for obtaining motor torque.
More precisely, the three Hall sensors are positioned on the stator of the motor so as to be spaced apart by 120° electrical, i.e. 60° mechanical for a motor having two pairs of electrical poles, as shown in FIG. 1, which gives an example arrangement for Hall effect sensors C1, C2, and C3 for a motor having four electrical poles A1 to A4, A1 and A3 being north magnetic poles and A2 and A4 being south magnetic poles.
The Hall effect sensors are sensitive to the polarity of the magnets of the rotor. Each sensor delivers a logic signal, which may have a “high” first value if the north magnet is facing the sensor, or else a “low” second value if the south magnet of polarity opposite to the north magnet is facing the sensor.
With the information delivered by these three Hall effect sensors, it is possible to reconstitute the position of the motor by creating electrical sectors of 60° electrical corresponding to various combinations of the three signals delivered by the sensors, as shown in FIG. 2. FIG. 2 is a graph plotting signals from the three Hall effect sensors C1 to C3 for the FIG. 1 motor having four electrical poles, i.e. two pairs of electrical poles, in which figure each vertical dashed line lies between two electrical sectors of 60° electrical.
Depending on the electrical sector, two switches of the inverter are controlled so as to cause current to flow that serves to provide motor torque. Such control is referred to as “120°” control.
The operation of an electric motor under 120° control can be represented by a so-called “Fresnel” diagram as shown in FIG. 3, where V is the voltage applied by the inverter to the terminals of the synchronous machine, E is the electromotive force (ems) of the synchronous machine, which is proportional to its speed of rotation, I is the current in a motor phase, R is the resistance of the winding of the motor, L is the inductance of the motor, and ω is the electrical angular frequency of the motor, given that ω can be expressed as a function of the electrical frequency of rotation of the motor in application of the equation ω=2πf.
The phase difference corresponding to the angle between the voltage V applied by the inverter and the emf E of the motor is referred to as δ, also known as the “lag” angle.
The lag angle δ can be controlled since the phase of the emf E is deduced from the information delivered by the Hall effect sensors and the phase of the voltage V is determined by the control of the inverter.
This lag angle δ may be selected so as to maximize torque. Under such circumstances, the emf E and the current I in a phase of the motor are in-phase, as shown in the Fresnel diagram of FIG. 4.
In various fans, it is known to determine, i.e. impose, the lag angle δ of a synchronous motor mechanically by offsetting the positioning of the Hall effect sensors relative to the axes of the coils B1 to B3 of the phases of the motor, as shown in FIG. 5. The inverter thus receives information from the sensors and changes its control signals as soon as a change of sector is detected.
Since the lag angle δ is determined mechanically, various mechanical tolerances will lead to inaccuracy in this angle, which inaccuracy increases when the diameter of the motor is small. Usually, dispersion in the lag angle δ is found to lie in the range ±5° for a lag angle of 15° electrical.
Unfortunately, it is necessary to have good accuracy for this lag angle for the motor-and-inverter combination to act in repetitive manner, in particular from the points of view of efficiency, harmonics, and performance.
Specifically, the value of the lag angle δ has multiple consequences on the performance of the inverter-and-motor combination. Firstly, concerning the appearance of currents flowing in the phases of the motor, the harmonic spectrum of the current in the phases is rich to a greater or lesser extent, giving rise to greater or lesser ripple in the torque of the motor. The lag angle δ also has an influence on the efficiency of power conversion (active power and reactive power) and also on the harmonic spectra rejected into the network.
Inaccuracy concerning the lag angle thus has an immediate impact on the reproducibility of the performance of the fan.
In order to obtain and maintain the desired accuracy, there are generally two main problems that are encountered.
The first problem, which is already mentioned above, comes from the fact that the various tolerances concerning the positioning of the mechanical parts relative to one another give rise to a tolerance concerning the lag angle δ that is very large, particularly when the diameter of the motor is small.
The second major problem stems from the fact that the lag angle δ is set mechanically, which means that it is optimized for only one operating point of the motor, which generally corresponds to the operating point at which the consumption of the fan is at a maximum.
With a constant lag angle δ it is not possible for the performance of the combination formed by the inverter and the motor to be optimized for all of the operating points of a synchronous motor, and in particular the motor of a fan.
Specifically, the modulus of the emf E varies linearly as a function of speed, whereas, and by way of example, for a synchronous motor of a fan, the modulus of the motor current varies with the square of the speed.
This second problem associated with optimizing the lag angle on a single operating point of the synchronous motor can lead to excess consumption by the motor at other operating points. This phenomenon is made correspondingly worse when the power of the motor increases.